The Spectral Irradiance Monitor (SIM) measures spectral solar irradiance (SSI) incident on a plane surface at the top of the atmosphere that is normal to the Sun.. The integrated irradiance contribution measured by SIM is approximately 97% of the total solar irradiance measured by the TIM instrument. The SIM instrument first flew on the LASP-led SORCE mission. SIM currently aboard TSIS-1 with improvements that are key requirements to understanding the contributions from varying spectral irradiance on climate. A SIM instrument is currently in development for TSIS-2, scheduled to launch in 2024.